(1) Field of the Invention
The present invention relates to a method for reducing the transmit power of a aircraft collision avoidance surveillance system and more particularly to a Traffic Alert and Collision Avoidance System (TCAS).
(2) Description of Related Art
It will be readily appreciated that operation of an aircraft in densely populated airspace is dangerous and requires a significant amount of the flight crew""s time and attention to avoid other aircraft. In early aviation history, the task of identifying other aircraft intruding into airspace near an aircraft was left to the crew and air traffic control (ATC). However, as the number of aircraft increased, the time and attention required to navigate the aircraft through densely populated airspace began to be overwhelming.
To assist flight crews and ATC, a variety of aircraft collision avoidance systems (ACAS) have been installed on military and commercial aircraft during the last forty years. Currently, the Traffic Alert and Collision Avoidance System (commonly referred to as TCAS) is deployed on military and most commercial aircraft. TCAS provides vertical collision avoidance advisories to the flight crews and operates in traffic densities of up to 24 aircraft within 5.0 square nautical miles (0.3 aircraft per square nmi). Using the advisories, flight crews can alter their flight paths to maximize the amount of separation at the closest point of approach (CPA) as computed by TCAS based on range and range rates.
It will be appreciated that with a large number of aircraft operating in congested airspace and with each aircraft broadcasting transmissions, many of these transmissions may occur simultaneously with other aircraft or with transmissions between one of the aircraft and ATC. Thus, interference limiting is a necessary part of the surveillance function. To ensure that TCAS does not create an unacceptably high fruit rate for ATC radar, multiple TCAS units within detection range of each other (approximately 30 nmi) are designed to limit their own transmissions based on the number of aircraft in congested airspace. Thus, as the number of TCAS units increase, the interrogation rate and correspondingly the power allocation for each of them, must decrease in order to prevent undesired interference with ATC. Therefore, every TCAS unit counts the number of other TCAS units within 30 nmi. This number is often referred to as the NTA. The NTA is used by each TCAS to limit interrogation rate and transmit power as necessary. The total transmit power allowed for TCAS or other airborne collision avoidance system surveillance is limited by regulation, specifically by the Radio Technical Commission for Aeronautics (RTCA) TCAS Minimum Operation Performance Standard (MOPS) number DO-185A.
Because of the variety of avionics, the TCAS system must be designed to operate with Mode S, Mode A and Mode C avionics. Mode S is a type of interrogation and transponder reply process. The Mode S reply messages contain aircraft address identification, altitude and other information. RTCA regulations specify that Mode S tracking interrogation power use a specified transmit power that is often more than required to successfully interrogate target aircraft. In theory using maximum power to interrogate all targets provides a margin of safety regardless of distance between aircraft or the associated threat level. However, in crowded airspace, maximum transmit power from a large number of aircraft generates interference and complicates ATC operation.
TCAS also utilizes Mode A/C interrogation, which is another type of interrogation. Mode C surveillance uses an interrogation sequence that is referred to as Whisper-Shout (WS) to minimize detected synchronous garble from the air traffic control radar beacon system (ATCRBS) transponders or other Mode C transponders. Mode A equipped aircraft respond to Mode C interrogation but do not provide altitude information and thus appear as a non-altitude reporting Mode C transponder.
The whisper-shout interrogation uses variable power levels and suppression pulses to identify the range of targets. Specifically, three different Mode C interrogation sequences or modes: 1) monitor; 2) medium aircraft density; and 3) high density are used for Mode C surveillance. TCAS automatically selects the interrogation sequence required to resolve or prevent potential interference based on the aircraft in the vicinity. Accordingly, it is important to know the number of TCAS-equipped aircraft present within the airspace. If transmit power restrictions are required, surveillance capabilities will degrade.
While the total transmit power restrictions are sufficient for commonly used TCAS surveillance protocols, the transmit power restrictions set by RTCA limit the effectiveness of the TCAS in high-density airspace and prohibit incorporation of new features. Further, transmit power restrictions limit extended surveillance ranges. By way of example, the prior art TCAS limits the range of surveillance on all quadrants of the aircraft in order to comply with the DO-185A power restrictions. Clearly, there are significant benefits to providing a surveillance system with increased situational awareness in all quadrants while complying with the power restrictions set by RTCA for TCAS. It should be apparent that increased situational awareness would enhance aircraft safety in crowded airspace and support military applications of TCAS.
When multiple aircraft are in a confined airspace, there will be more frequent TCAS interrogations as each aircraft attempts to interrogate and track intruders. Thus, a method is needed that reduces the amount of transmit power when the NTA is high so that the RTCA transmit power restrictions are observed. What is also required is a method that efficiently uses transmit power to improve surveillance in terms of an extended range in crowded high-density airspace.
The present invention relates to an aircraft collision avoidance system and more particularly to a method for efficient use of the transmit power available to an aircraft using the Traffic Alert and Collision Avoidance System (TCAS). Efficient use of transmit power allows enhanced surveillance while simultaneously remaining within the transmit power restrictions set by the Radio Technical Commission for Aeronautics (RTCA) for TCAS II as specified in (MOPS) DO-185A.
In one preferred embodiment, operation of prior art TCAS is modified during Mode S broadcasts transmit at reduced power levels during the interrogation phase. More specifically, the present invention comprises a novel method for reducing Mode S tracking interrogation power as a function of range. Once traffic (an aircraft within the surveillance volume surrounding a reference aircraft) is identified, Mode S tracking interrogation is attempted using less power than what is currently specified by RTCA regulations. In accordance with the present invention, interrogation power reduction is reduced while providing surveillance of Mode S traffic. The present method is particularly useful when the other aircraft is sufficiently far away such it is not currently classified as a threat intruder and the flight crew is not currently required to begin taking evasive action.
In another embodiment, a variable power density whisper-shout interrogation technique is employed that minimizes both transmit power and the potential for garbling during Mode A/C interrogation. Mode C interrogation refers to a type of ATCRBS interrogation that requires a transponder to transmit its host aircraft""s present altitude. The Mode C response to an interrogation contains information regarding the altitude encoded in a plurality of pulses positioned between framing pulses. Mode A refers to a type of ATCRBS interrogation that requires an aircraft transponder to transmit a selected identification code but without altitude information. Whisper-Shout is one method of controlling garble from the air traffic control radar beacon system (ATCRBS) transponders through the combined use of variable power levels and suppression pulses. Garble is a well-understood concept in the art. When the reference aircraft encounters two or more TCAS aircraft, a TCAS aircraft and a ground site or two ground sites in a multi-site environment with overlapping surveillance areas, the environment may generate non-synchronous garbling. Synchronous garble is the erroneous, distorted or lost data caused when two or more aircraft are interrogated within a few microseconds of each other. This is most often caused by replies to the interrogation overlapping in time from vertically separated aircraft.
Prior art TCAS specifies three different Mode C interrogation sequences: 1) monitor; 2) medium-density; and 3) high-density. During the Mode C interrogation sequence, TCAS automatically selects the interrogation sequence required to resolve or prevent potential garbling.
The present invention discloses a method that combines characteristics of the medium and high-density Mode C interrogation sequences with variable transmit power control such that increased power is only applied within a beam when required to alleviate potential or observed garbling. If garbling is observed during a medium whisper-shout, the method of the present invention attempts to clear the garbling by using focused high-density whisper-shout steps but only in the ranges where garbling was detected.
In yet another embodiment of the present invention, a special E-TCAS Broadcast message is used to provide accurate NTA counts to E-TCAS equipped aircraft that are flying in formation. A broadcast inhibit is currently employed by aircraft flying in formation to minimize TCAS transmit power. By minimizing the count of aircraft, commercial aircraft need not track the formation members and TCAS will not unnecessarily determine that the airspace has a high number of TCAS units that require a reduction in surveillance rate and range. More specifically in E-TCAS, formation members will not transmit UF 16 TCAS broadcast. Thus, commercial aircraft will not unnecessarily reduce their broadcast power because of an apparent high aircraft density in the airspace. The formation members will transmit a UF 19 E-TCAS broadcast so that other E-TCAS equipped military aircraft will always have an accurate count of aircraft density. UF 19 E-TCAS is the broadcast protocol reserved for military aircraft. Receiving E-TCAS aircraft, in accordance with the present invention, will use the UF 19 TCAS in generating an NTA count, but the surveillance range for commercial TCAS units in the area will not be affected by the presence of the formation.
In yet another embodiment of the present invention, aircraft flying in formation minimize broadcast power by sharing information between the formation leader and the other aircraft in the formation. Accordingly, transmit power is minimized even though there are significant numbers of aircraft in the area. More specifically, one method of the present invention provides for the formation leader to perform active surveillance with 360-degree range coverage. The formation leader data-links the contents of its TCAS intruder file to other members of the formation. Formation members use passive surveillance to track the position of the formation leader as well as the positions of the other ADS-B equipped intruders in the vicinity. ADS-B refers to a type of aircraft surveillance that utilizes global positioning system (GPS) data to track traffic within the surveillance volume.
The formation leader uses the UF/DF 19 data-link to exchange the following data to other aircraft in the formation: time stamp, intruder""s position, bearing, altitude and Mode A identifier. Mode A refers to a type of ATCRBS interrogation that requires an aircraft transponder to transmit a selected identification code but does not transmit data in the altitude field. Only intruders that are obtained via the formation leader""s active surveillance will be data-linked to other aircraft in the formation so as to minimize transmit power from the formation.
These and other advantages of the present invention not specifically described above will become clear within the detailed discussion herein.